1. Field of the Invention
This invention relates to improvements in methods and apparatus for electrostatic image development and, more particularly, to controlling toner developed mass in an electrophotographic apparatus and electrostatic printing apparatus.
2. Background
For proper understanding of the invention and the disclosure, the following basic definitions are provided.
A "print sheet" is a paper which has been fused with a toner image in an electrostatic device.
DMA is an abbreviation for developed toner mass per unit area of toner on a print sheet, and is usually given in the units of mg/cm.sup.2. DMA refers to the actual amount of toner solids per unit area of paper.
Gloss is a measure of an image's shininess which should be measured after a toner image has been fused onto a print sheet, since the fusing process alters the gloss. It is defined at a specular angle, which is the angle between the perpendicular to a surface and the reflected ray that is numerically equal to the angle of incidence and that lies in the same plane as the incident ray and the perpendicular but opposite to the incident ray. The choice of a specular angle for desired gloss characteristics is determined by the nature of the substrate. The specular angle is usually increased as the substrate gloss decreases. Thus, for low to medium gloss substrates, best results are obtained with relatively high specular angles.
Image reflectance density (hereinafter "density") corresponds to color strength in that more intense color appears denser. Density is measured using a reflection densitometer whereupon image gloss reduces the amount of light that reaches the detector of the densitometer. The densitometer interprets the reduction of light as increased absorption of incident light and thus higher color density. When all other factors are equal, glossier images appear denser. Thus, comparing density values when gloss is changing can cause erroneous results.
A typical electrophotographic printing machine employs a photoconductive member that is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon, in the irradiated areas, to record an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. The development field, which causes image development, is the electric field between the image charge and a development electrode that is grounded or electrically biased. Two types of developer materials are typically employed in electrophotographic reproducing machines. One type of developer material is known as a dry developer material and comprises carrier granules having toner particles adhering triboelectrically thereto. Alternatively, the developer material may be a liquid material comprising a liquid carrier having pigmented particles dispersed therein. In either case, the image recorded on the photoconductive member is developed and transferred to a sheet of support material. Thereafter, the developed image on the sheet of support material is heated to permanently fuse it thereto.
The process control system of a toner imaging device can use a feedback loop to control image reflectance density. Image reflectance density is measured and used to adjust toner development parameters, such as the development field, to obtain a desired reflectance density on subsequent prints and to maintain the toner DMA in a desired range. If the toner DMA is too high then images can become smeared in subsequent steps and if DMA is too low fine image features can remain undeveloped. However, reflectance density is often not a direct function of development field and in some cases when image gloss is uncontrolled, reflectance density can decrease with an increasing development field and vice versa. An improved process control system would measure and use toner DMA to adjust toner development parameters since toner DMA is directly related to development field over a broad range of development fields. However, a problem arises in that toner developed mass is difficult to measured directly in an imaging device.
The prior art does not recognize the problem that the development field of an electrostatic imaging device can be adjusted by determining the toner developed mass per unit area as related to a combination of reflectance density and image gloss. In the past, reflectance density has been measured by densitometers to provide a means for toner concentration control. However, no process control system in an electrostatic device has utilized or suggested the process of measuring both reflectance density and gloss to adjust a development field using a toner developed mass which is functionally related to both reflectance density and image gloss. The above problems in the prior art are prevalent in electrostatic devices which use either liquid toner or toner powder.
3. Description of the Related Art
The following references demonstrate the teachings of the prior art, but none of the references recognizes the effect of the image gloss on the toner density requirements.
U.S Pat. No. 4,551,004 to Paraskevopoulos describes an apparatus for monitoring toner concentration on a photoreceptor surface by optically sensing the amount of toner that is triboelectrically attracted to a portion of the photoreceptor surface. The toner sensor of the apparatus acts as a densitometer for determining the density of the toner on the photoreceptor surface. The apparatus includes a light emitting diode, a phototransistor, a beam splitter, and a lens disposed between the beam splitter and the photoreceptor surface to collimate the light beam between the lens and the photoreceptor surface. A portion of the light emitted from the LED is transmitted through the beam splitter and the lens to the photoreceptor surface. Collimated light is reflected from the photoreceptor surface back through the lens and reflected from the beam splitter to the phototransistor. The output signal from the phototransistor is thus independent of the distance of the lens from the photoreceptor surface.
U.S. Pat. No. 4,572,654 to Murai et al describes a method for electrophotographic image density control which controls at least one of various image density parameters in response to detected values of different pattern areas. The image density parameters include an amount of charge deposited on a photoconductive element by a charger, bias voltage for development, toner density in a developer, amount of toner supplied to a developing unit and transfer potential. At least two pattern areas having different potentials are formed on the surface of the photoconductive element by at least one of various means for forming charge patterns which include controlling the energization of the charger, controlling an illuminating lamp and projecting an image pattern. At each of the pattern areas, at least one of the values associated with the image density is detected which includes a surface potential of the pattern area before development, a toner density of the pattern area after development, surface potential of the pattern area after development, and image density of an area of a transferred image which corresponds to the pattern area. The value associated with a predetermined value is compared and matched to one specific pattern area.
U.S. Pat. No. 4,829,336 to Champion et al discloses a patch sensing toner concentration control method and apparatus in which the optical density of reproduction output can be changed without changing the quantity of toner that is deposited on the photoconductor's test patch area. The test patch area receives toner as the patch area passes through a developer station under the influence of a patch development electrical field or vector. Light that is reflected from a bare photoconductor area is compared to light that is reflected from a toned test patch area. The ratio of these two reflected light intensities is used to control the addition of toner to the developer station. Optical density of the reproduction output is changed by changing the toner concentration in the developer station. The toner concentration control method and apparatus of the invention is constructed and arranged to require a fixed or constant ratio of light reflection as an indication of proper toner concentration, independent of the absolute value of toner concentration. Toner concentration, and thereby optical density of reproduction output, is changed by changing the magnitude of the patch development vector, while maintaining the reproduction development vector constant.
U.S. Pat. No. 4,179,213 to Queener describes a method for improving the quality of an electrophotographic image by controlling the toner concentration, the image voltage of the photoconductor, and the bias voltage on the developer. The method pins the value of a white, gray or otherwise colored, single-shaded vector, where the vector represents the value of the image voltage minus the developer voltage. Valuation of changes in the image voltage are obtained by: (1) sensing the reflectivity of a developed single-shaded image and converting that into a representative voltage; (2) sensing the reflectivity of the bare photoconductor and converting that into a representative voltage; (3) obtaining a comparison of the representative image and reference voltages; and (4) noting changes in the comparison. Pinning the vector calls for adjusting the member, for producing the vector (such as the developer voltage or document illumination intensity level), an amount necessary to compensate for the change in the image voltage.
U.S. Pat. No. 4,337,338 to Ernst describes a method and apparatus for copier quality monitoring and control where data correlating to the light reflectance of a maximum toned area and a minimum toned area is recorded to establish measurement standards. A test pattern is imaged onto the photoconductor by controlled illumination levels in a series of steps with the detection of light reflectance from the test pattern being subsequently compared to establish the maximum black and maximum white criteria for storage. Light reflected from cleaned photoconductor areas and subsequently established toner patches then is compared with original test pattern reflectance data to provide a basis for toner replenishment and machine function monitoring.
U.S. Pat. No. 4,312,589 to Brannan et al. describes an electrophotographic copier having a tone concentration control apparatus which periodically measures, by light reflectance, the optical density of toner deposited on a photoconductor test area. As the results of the toner patch test cycle indicate lower than acceptable toner density, as by high light reflectance off the test patch, the photoconductor's charge magnitude is periodically increased until a working charge magnitude is reached. The results of the toner patch test cycle are operable to add toner to the copier's developer only when the photoconductor's charge magnitude has been increased to be approximately equal to the working magnitude.
U.S. Pat. No. 4,466,731 to Champion et al. describes an electrophotographic machine and method with high density toner concentration control. A toner concentration control test cycle is run with a test patch produced, preferably in the area of the photoconductor ordinarily used for document reproduction. The optical reflectivity of the developed test area is sensed and the result used to replenish toner if indicated.